The more advanced Decca moving map system was called Danac, which, iirc, used a map that bore some intuitive relationship to ground features. It was automatic in use - but you still had to load the right map rolls for the trip you were about to do and tell it where to start from. Sometimes on longer or triangular trips that ran off the map a new roll could be loaded in flight.
The basic Decca setup was a nightmare. The map was a closely packed mass of intersecting parabolic lines in three colours and frequently lost lock and had to be reset - a procedure that took some learning. This involved reference to the "Deccometers", three instruments reporting lane number in each colour, and one master instrument containing "spiders" and "cheeses" - wire crosses instead of needles that divided the instrument into 6 (?) sectors, and something that looked like a slice of cheese. Somehow, by base cunning and a great deal of luck one might sometimes be able to interepret the complex lane indicators and wildly gyrating spiders and cheeses and figure out which lane and fraction thereof you were on (in each colour) and then follow each coloured parabola to find where they all intersected on the map, by which time you'd flown a mile or three and weren't where you'd calculated any more, and had to start all over again. When it was working it was quite good - accuracy enough for non-precision approaches to 3-400ft, perhaps as good as 50m laterally if the lanes intersected closely and you weren't too far from the base stations, certainly not much worse than 100 - 150m over most of the UK and coastal waters. Accuracy dropped off as intersection angles of the lanes became more acute, and/or the lanes spread out at longer range but was useable up to maybe 600 miles from the base stations - I'd be guessing but accuracy of perhaps a mile or more out there. The real nasty gotcha was that when it "jumped" a lane (or more) it would carry on without a hiccup giving no indication that it was now lying like a cheap chinese watch, so had to be constantly monitored...a dreadful labour-intensive nightmare in a 2 crew cockpit at 500', 2 mile vis and blowing a gale as so often on the N Sea, with rigs lurking in the murk not far ahead. Still, some people seemed to be able to work it with ease. The Danac was fine.
(Fuzzy memory syndrome active here as it was in the mid '80s!)
Far, far better - infinitely better, was the Loran (or maybe a VLF/Omega) that one of our machines had. Looked and acted like an early GPS, ran automatically off gound stations on the other side of the world and was nearly as accurate as Decca. Why didn't everyone use that? And why did Britain persist with the antedelluvian Decca?
Decca Navigator - Airborne Receivers and Indicators
The systems I've tried to describe are, I think, the Mk8 and the Mk15 with DANAC which are well depicted at the site above. The Mk19 section also shows good pics of a DANAC chart reader.
http://www.jproc.ca/hyperbolic/decca...ice_map_b1.jpg
This map is a great deal less cluttered than the ones actually used, for a start it only shows two colours but gives the general idea.
Decca Navigator - System Overview
Chapter and verse here.